Solar cell

ABSTRACT

A solar cell is discussed. The solar cell includes a substrate of a first conductive type, an emitter region which is positioned at a front surface of the substrate and has a second conductive type different from the first conductive type, a front passivation region including a plurality of layers which are sequentially positioned on the emitter region, a back passivation region which is positioned on a back surface opposite the front surface of the substrate and includes three layers, a plurality of front electrodes which pass through the front passivation region and are connected to the emitter region, and at least one back electrode which passes through the back passivation region and is connected to the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending U.S. application Ser. No.13/543,135, filed on Jul. 6, 2012, which claims priority to and thebenefit of Korean Patent Application No. 10-2011-0139066 filed in theKorean Intellectual Property Office on Dec. 21, 2011. The entirecontents of all these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a solar cell.

2. Description of the Related Art

Recently, as existing energy sources such as petroleum and coal areexpected to be depleted, interests in alternative energy sources forreplacing the existing energy sources are increasing. Among thealternative energy sources, solar cells for generating electric energyfrom solar energy have been particularly spotlighted.

A solar cell generally includes semiconductor parts, which respectivelyhave different conductive types, for example, a p-type and an n-type andthus form a p-n junction, and electrodes respectively connected to thesemiconductor parts of the different conductive types.

When light is incident on the solar cell, electron-hole pairs includingelectrons and holes as carriers are produced in the semiconductor parts.The carriers move to the n-type semiconductor part and the p-typesemiconductor part under the influence of the p-n junction. Namely, theelectrons move to the n-type semiconductor part, and the holes move tothe p-type semiconductor part. Then, the electrons and the holes arecollected by the different electrodes respectively connected to then-type semiconductor part and the p-type semiconductor part. Theelectrodes are connected to each other using electric wires to therebyobtain electric power.

SUMMARY OF THE INVENTION

In one aspect, there is a solar cell including a substrate of a firstconductive type, an emitter region which is positioned at a frontsurface of the substrate and has a second conductive type different fromthe first conductive type, a front passivation region including aplurality of layers which are sequentially positioned on the emitterregion, a back passivation region which is positioned on a back surfaceopposite the front surface of the substrate and includes three layers, aplurality of front electrodes which pass through the front passivationregion and are connected to the emitter region, and at least one backelectrode which passes through the back passivation region and isconnected to the substrate, wherein the plurality of the backpassivation region include a first back passivation layer which ispositioned on the back surface of the substrate and is formed of siliconoxide, a second back passivation layer which is positioned on the firstback passivation layer and is formed of silicon nitride, and a thirdback passivation layer which is positioned on the second backpassivation layer and is formed of aluminum oxide.

The first back passivation layer may have a thickness of about 5 nm to30 nm, and the third back passivation layer may have a thickness ofabout 5 nm to 20 nm.

The first back passivation layer and the third back passivation layermay have the same thickness.

A thickness of the second back passivation layer may be greater than athickness of the first back passivation layer and a thickness of thethird back passivation layer.

Each of the first back passivation layer and the third back passivationlayer may have the thickness of about 10 nm, and the second backpassivation layer may have the thickness of about 90 nm.

The first back passivation layer may have a thickness of about 5 nm to30 nm, the second back passivation layer may have a thickness of about10 nm to 200 nm, and the third back passivation layer may have athickness of about 5 nm to 20 nm.

The plurality of layers of the front passivation region may include analuminum oxide layer which is positioned on the emitter region and isformed of aluminum oxide, and a silicon nitride layer which ispositioned on the aluminum oxide layer and is formed of silicon nitride.

The aluminum oxide layer may have a thickness of about 5 nm to 20 nm,and the silicon nitride layer may have a thickness of about 10 nm to 200nm.

A thickness of the silicon nitride layer may be greater than a thicknessof the aluminum oxide layer.

The silicon nitride layer may have the thickness of about 90 nm, and thealuminum oxide layer may have the thickness of about 10 nm.

The plurality of layers of the front passivation region may furtherinclude a silicon oxide layer which is positioned between the emitterregion and the aluminum oxide layer and is formed of silicon oxide.

The silicon oxide layer may have a thickness of about 5 nm to 30 nm.

Each of the silicon oxide layer and the aluminum oxide layer may have athickness of about 10 nm, and the silicon nitride layer may have athickness of about 90 nm.

The silicon oxide layer and the first back passivation layer may havethe same thickness.

The first conductive type may be an n-type, and the second conductivetype may be a p-type.

The aluminum oxide layer and the third back passivation layer may havethe same thickness.

The at least one back electrode may include one back electrode having aplurality of contact portions which locally abut the back surface of thesubstrate. The solar cell may include a plurality of back surface fieldregions respectively positioned at the back surface of the substrateabutting the plurality of contact portions.

The at least one back electrode may include a plurality of backelectrodes which locally abut the back surface of the substrate and areseparated from one another. The solar cell may include a back surfacefield region which is positioned at the back surface of the substrateand abuts the plurality of back electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a partial perspective view of a solar cell according to anexample embodiment of the invention;

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1; and

FIG. 3 is a partial cross-sectional view of a solar cell according toanother example embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. It should be noticedthat a detailed description of known arts will be omitted if suchdescriptions of the known arts could obscure understanding of theembodiments of the invention.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. It should be understood that when anelement such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

Example embodiments of the invention will be described with reference toFIGS. 1 to 3.

A solar cell according to an example embodiment of the invention isdescribed in detail with reference to FIGS. 1 and 2.

As shown in FIG. 1, a solar cell 11 according to an example embodimentof the invention includes a substrate 110, an emitter region 121positioned at an incident surface (hereinafter, referred to as “a frontsurface or a first surface”) of the substrate 110 on which light isincident, a front passivation region (or a first passivation region) 191positioned directly on the emitter region 121 (i.e., on the frontsurface of the substrate 110), a back passivation region (or a secondpassivation region) 192 positioned directly on a back surface (or asecond surface) opposite the front surface of the substrate 110, a frontelectrode part 140 which is positioned on the front surface of thesubstrate 110 and is connected to the emitter region 121, a backelectrode part 150 which is positioned on the back surface of thesubstrate 110 and is connected to the substrate 110, and a plurality ofback surface field regions 172 selectively (or locally) positioned onthe back surface of the substrate 110.

The substrate 110 is a semiconductor substrate formed of a semiconductorsuch as first conductive type silicon, for example, n-type silicon,though not required. The semiconductor used in the substrate 110 is acrystalline semiconductor, such as single crystal silicon andpolycrystalline silicon.

When the substrate 110 is of an n-type, the substrate 110 is doped withimpurities of a group V element such as phosphorus (P), arsenic (As),and/or antimony (Sb). Alternatively, the substrate 110 may be of ap-type and/or may be formed of a semiconductor material other thansilicon. If the substrate 110 is of the p-type, the substrate 110 may bedoped with impurities of a group III element such as boron (B), gallium(Ga), and/or indium (In).

As shown in FIGS. 1 and 2, a separate texturing process is performed ona flat front surface of the substrate 110 to form a textured surfacecorresponding to an uneven surface having a plurality of protrusions anda plurality of depressions or having uneven characteristics. In thisinstance, the emitter region 121 and the front passivation region 191positioned on the front surface of the substrate 110 have the texturedsurface.

As described above, because the front surface of the substrate 110 istextured, an incident area of the substrate 110 increases and a lightreflectance decreases due to a plurality of reflection operationsresulting from the textured surface. Hence, an amount of light incidenton the substrate 110 increases, and the efficiency of the solar cell 11is improved.

As shown in FIGS. 1 and 2, maximum diameters D1 and maximum heights D2of the plurality of protrusions of the textured surface of the substrate110 are non-uniform. Therefore, the plurality of protrusions have thedifferent maximum diameters D1 and the different maximum heights D2.

The emitter region 121 positioned at the front surface of the substrate110 is an impurity doped region doped with impurities of a secondconductive type (for example, p-type) opposite the first conductive type(for example, n-type) of the substrate 110. Thus, the emitter region 121of the second conductive type forms a p-n junction along with a firstconductive type region of the substrate 110.

Regarding the carriers, for example, electrons and holes produced bylight incident on the substrate 110, the electrons and the holesrespectively move to the n-type semiconductor and the p-typesemiconductor by a built-in potential difference resulting from the p-njunction between the substrate 110 and the emitter region 121. Thus,when the substrate 110 is of the n-type and the emitter region 121 is ofthe p-type, the electrons move to the substrate 110 and the holes moveto the emitter region 121.

The front passivation region 191 positioned on the emitter region 121includes a first front passivation layer 1911 positioned directly on theemitter region 121, a second front passivation layer 1912 positioned onthe first front passivation layer 1911, and a third front passivationlayer 1913 positioned on the second front passivation layer 1912.

The first front passivation layer 1911 is formed of silicon oxide(SiOx), in particular, a thermal oxide, for example, silicon dioxide(SiO₂). The first front passivation layer 1911 has a thickness of about5 nm to 30 nm and a refractive index of about 1.5.

The second front passivation layer 1912 is formed of aluminum oxide(Al₂O₃). The second front passivation layer 1912 has a thickness ofabout 5 nm to 20 nm and a refractive index of about 1.7.

The third front passivation layer 1913 is formed of hydrogenated siliconnitride (SiNx:H). The third front passivation layer 1913 has a thicknessof about 10 nm to 200 nm and a refractive index of about 1.5 to 3.

In the embodiment of the invention, the first front passivation layer1911 and the second front passivation layer 1912 may have the samethickness, and the third front passivation layer 1913 may have athickness greater than the first and second front passivation layers1911 and 1912. For example, the thickness of each of the first andsecond front passivation layers 1911 and 1912 may be about 10 nm, andthe thickness of the third front passivation layer 1913 may be about 90nm.

The front passivation region 191 having the three layers performs apassivation function which converts a defect, for example, danglingbonds existing at and around the front surface of the substrate 110 intostable bonds to thereby prevent or reduce a recombination and/or adisappearance of carriers moving to the surface of the substrate 110.

Because the first front passivation layer 1911 formed of a thermal oxidehaving the most passivation effect is formed as a lowermost layer of thefront passivation region 191 directly adjoining the front surface of thesubstrate 110, the passivation effect of the front passivation region191 is improved by the thermal oxide of the first front passivationlayer 1911.

In the embodiment of the invention, when the substrate 110 is of then-type, the emitter region 121 positioned at the front surface of thesubstrate 110 is of the p-type. Further, the first front passivationlayer 1911, which is formed of silicon oxide and has characteristic ofpositive fixed charges, is positioned directly on the emitter region121. Thus, because positive charges (i.e., holes) moving to the emitterregion 121 have the same polarity as the first front passivation layer1911, the positive charges (i.e., holes) may be pushed to the backsurface of the substrate 110 due to the polarity of the first frontpassivation layer 1911.

It is preferable, but not required, that the first front passivationlayer 1911 may have a minimum thickness capable of performing thepassivation effect, so as to prevent an adverse influence of the firstfront passivation layer 1911 formed of silicon oxide. Hence, asdescribed above, the first front passivation layer 1911 may have thethickness of about 5 nm to 30 nm.

When the thickness of the first front passivation layer 1911 is equal toor greater than about 5 nm, the first front passivation layer 1911 isuniformly coated on the emitter region 121 and more smoothly performsthe passivation function. When the thickness of the first frontpassivation layer 1911 is equal to or less than about 30 nm, the firstfront passivation layer 1911 formed of silicon oxide does not adverselyaffect the movement of carriers to the emitter region 121. Further, amanufacturing time and a manufacturing cost of the first frontpassivation layer 1911 are reduced.

In general, aluminum oxide (Al₂O₃) has the characteristic of strongnegative fixed charges. Thus, the second front passivation layer 1912formed of aluminum oxide has the characteristic of strong negative fixedcharges. Hence, the strong negative fixed charges of the second frontpassivation layer 1912 compensate for an influence of weak positivefixed charges of the first front passivation layer 1911 which underliesthe second front passivation layer 1912 and adversely affects holesmoving to the emitter region 121.

In addition, an intensity of the negative fixed charges of the secondfront passivation layer 1912 is much greater than an intensity of thepositive fixed charges of the first front passivation layer 1911.Therefore, holes moving to the emitter region 121 are drawn to thesecond front passivation layer 1912 because of an influence of thenegative fixed charges of the second front passivation layer 1912.Further, electrons moving to the emitter region 121 are pushed to theback surface of the substrate 110 because of the influence of thenegative fixed charges of the second front passivation layer 1912.

As described above, when the substrate 110 is of the n-type, arecombination and/or a disappearance of electrons and holes areprevented or reduced and also an amount of carriers moving to theemitter region 121 and the back surface of the substrate 110 furtherincreases because of the negative fixed charges of the second frontpassivation layer 1912.

As described above, the second front passivation layer 1912 has athickness of about 5 nm to 20 nm. When the thickness of the second frontpassivation layer 1912 is equal to or greater than about 5 nm, thesecond front passivation layer 1912 is uniformly coated on the firstfront passivation layer 1911, stably performs the passivation function,and stably holds the intensity of the negative fixed charges. When thethickness of the second front passivation layer 1912 is equal to or lessthan about 20 nm, increases in the manufacturing cost and manufacturingtime caused by an unnecessary increase in the thickness of the secondfront passivation layer 1912 are prevented or reduced.

Further, when the second front passivation layer 1912 formed of aluminumoxide performing the passivation function by use of oxygen is exposed tooxygen, the passivation effect of the second front passivation layer1912 using aluminum oxide is greatly reduced. Therefore, the third frontpassivation layer 1913 formed of hydrogenated silicon nitride ispositioned on the second front passivation layer 1912.

Thus, the third front passivation layer 1913 serves as a capping layerfor protecting the second front passivation layer 1912.

The third front passivation layer 1913 performs the passivation functionby use of hydrogen. More specifically, the third front passivation layer1913 moves hydrogen contained in the third front passivation layer 1913to the front surface of the substrate 110 in a thermal process forforming the front electrode part 140, thereby performing the passivationfunction.

Accordingly, the third front passivation layer 1913 is formed as anuppermost layer of the front passivation region 191, so that the thirdfront passivation layer 1913 serves as the capping layer of the secondfront passivation layer 1912 at the front surface of the substrate 110and improves the passivation effect at the front surface of thesubstrate 110.

As described above, because the front passivation region 191 having themulti-layered structure (for example, the three layers) is formed on thefront surface of the substrate 110 to perform the passivation function,the passivation effect at the front surface of the substrate 110 isfurther improved.

In the embodiment of the invention, the front passivation region 191serves as an anti-reflection region capable of minimizing the reflectionof light incident on the front surface of the substrate 110, therebyincreasing an amount of light incident on the substrate 110.

Because the second front passivation layer 1912 is positioned on thefirst front passivation layer 1911 having the refractive index less thanthe second front passivation layer 1912, a refractive index in goingfrom air (having a refractive index of about 1) to the substrate 110(having a refractive index of about 3.4) gradually increases. Thus, thefirst and second front passivation layers 1911 and 1912 may have as thina thickness as possible, so as to prevent or minimize a reduction in ananti-reflection effect of light resulting from a reversion of therefractive index and to smoothly obtain the passivation effect. Hence,as described above, each of the first and second front passivationlayers 1911 and 1912 may have the thickness of about 10 nm, for example.

The front electrode part 140 includes a plurality of front electrodes141 and a plurality of front bus bars 142 connected to the plurality offront electrodes 141.

The plurality of front electrodes 141 are connected to the emitterregion 121 and are separated from one another. The plurality of frontelectrodes 141 extend parallel to one another in a fixed direction. Theplurality of front electrodes 141 collect carriers (for example, holes)moving to the emitter region 121.

The plurality of front bus bars 142 are connected to the emitter region121 and extend parallel to one another in a direction crossing the frontelectrodes 141.

In this instance, the front bus bars 142 are positioned on the samelevel layer as the front electrodes 141 and are electrically andphysically connected to the front electrodes 141 at crossings of thefront electrodes 141 and the front bus bars 142.

Accordingly, as shown in FIG. 1, the plurality of front electrodes 141have a stripe shape extending in a transverse (or longitudinal)direction, and the plurality of front bus bars 142 have a stripe shapeextending in a longitudinal (or transverse) direction. Hence, the frontelectrode part 140 has a lattice shape on the front surface of thesubstrate 110.

The front bus bars 142 have to collect not only carriers (for example,holes) moving from the emitter region 121 but also carriers collected bythe front electrodes 141 crossing the front bus bars 142 and have tomove the collected carriers in a desired direction. Thus, a width ofeach front bus bar 142 is greater than a width of each front electrode141.

The front bus bars 142 are connected to an external device and outputthe collected carriers to the external device.

The front electrode part 140 including the front electrodes 141 and thefront bus bars 142 is formed of at least one conductive material such assilver (Ag).

In the embodiment of the invention, the number of front electrodes 141and the number of front bus bars 142 may vary, if necessary or desired.

As shown in FIGS. 1 and 2, the front electrode part 140 including thefront electrodes 141 and the front bus bars 142 passes through the frontpassivation region 191 of the multi-layered structure positioned on thefront surface of the substrate 110 and is connected to the emitterregion 121. Hence, the front passivation region 191 including the firstto third front passivation layers 1911 to 1913 is not positioned in aformation area of the front electrode part 140 in the front surface ofthe substrate 110. Namely, the first to third front passivation layers1911 to 1913 are positioned between the front electrodes 141 and betweenthe front bus bars 142.

The back passivation region 192 positioned on the back surface of thesubstrate 110 may include three passivation layers in the same manner asthe front passivation region 191.

More specifically, the back passivation region 192 includes a first backpassivation layer 1921 which is positioned directly on the back surfaceof the substrate 110 and is formed of silicon oxide (SiOx), inparticular, a thermal oxide, for example, silicon dioxide (SiO₂), asecond back passivation layer 1922 which is positioned on the first backpassivation layer 1921 and is formed of hydrogenated silicon nitride(SiNx:H), and a third back passivation layer 1923 which is positioned onthe second back passivation layer 1922 and is formed of aluminum oxide(Al₂O₃).

The first back passivation layer 1921 may be formed along with the firstfront passivation layer 1911. In this instance, the first backpassivation layer 1921 may have the same layer characteristic as thefirst front passivation layer 1911. Further, the third back passivationlayer 1923 may be formed along with the second front passivation layer1912, and thus, may have the same layer characteristic as the secondfront passivation layer 1912. Thus, the first back passivation layer1921 may have the same thickness and the same composition as the firstfront passivation layer 1911, and the third back passivation layer 1923may have the same thickness and the same composition as the second frontpassivation layer 1912.

The first back passivation layer 1921 has a thickness of about 5 nm to30 nm and a refractive index of about 1.5. The third back passivationlayer 1923 has a thickness of about 5 nm to 20 nm and a refractive indexof about 1.7.

The second back passivation layer 1922 has a thickness of about 10 nm to200 nm and a refractive index of about 1.5 to 3 in the same manner asthe third front passivation layer 1913.

The first back passivation layer 1921 formed of silicon oxide and thethird back passivation layer 1923 formed of aluminum oxide may have thesame thickness, and the second back passivation layer 1922 formed ofhydrogenated silicon nitride may have the thickness greater than thefirst and third back passivation layers 1921 and 1923, in the samemanner as the front passivation region 191.

Since effects obtained when each of the first to third back passivationlayers 1921 to 1923 satisfies the above-described thickness range arethe same as the first to third front passivation layers 1911 to 1913, afurther description may be briefly made or may be entirely omitted.

Further, the first to third back passivation layers 1921 to 1923 performthe same function as the first to third front passivation layers 1911 to1913.

More specifically, the passivation effect of the back passivation region192 is improved by the first back passivation layer (i.e., the thermaloxide layer) 1921, which is positioned directly on the back surface ofthe substrate 110 and is formed as a lowermost layer of the backpassivation region 192. Further, the passivation effect of the backpassivation region 192 is further improved because of hydrogen containedin the second back passivation layer (i.e., the hydrogenated siliconnitride layer) 1922 positioned on the first back passivation layer 1921.

Electrons moving to the back surface of the substrate 110 are drawn tothe back surface of the substrate 110 because of the first and secondback passivation layers 1921 and 1922 each having the characteristic ofpositive fixed charges. Further, holes moving to the back surface of thesubstrate 110 are pushed to the emitter region 121 positioned at thefront surface of the substrate 110. Because the third back passivationlayer (i.e., the aluminum oxide layer) 1923 having the characteristic ofstrong negative fixed charges is formed as an uppermost layer of theback passivation region 192, the negative fixed charges of the thirdback passivation layer 1923 are blocked or shielded by the first andsecond back passivation layers 1921 and 1922 underlying the third backpassivation layer 1923. Hence, an adverse influence of the third backpassivation layer 1923 on electrons moving to the back surface of thesubstrate 110 is prevented or reduced.

Accordingly, the back passivation region 192 including the three layersimproves the passivation effect in the back surface of the substrate110. Further, the back passivation region 192 further improves a fieldpassivation effect using the fixed charges because of the dispositionorder of layers different from the front passivation region 191.

Because the back passivation region 192 including the three layers isformed on the back surface of the substrate 110, light passing throughthe substrate 110 reflected by the back passivation region 192 and isre-incident on the substrate 110. Thus, an amount of light incident onthe substrate 110 further increases.

The front passivation region 191 and the back passivation region 192 maybe formed using various methods.

For example, the emitter region 121 is formed at the substrate 110, andthen silicon oxide (for example, silicon dioxide (SiO₂)) is coated oneach of the front surface (i.e., on the emitter region 121) and the backsurface of the substrate 110 using a furnace to simultaneously form thefirst front passivation layer 1911 and the first back passivation layer1921. Subsequently, hydrogenated silicon nitride (SiNx:H) is stacked onthe first back passivation layer 1921 of the substrate 110 using aplasma enhanced chemical vapor deposition (PECVD) method to form thesecond back passivation layer 1922.

Next, aluminum oxide (Al₂O₃) is stacked on the first front passivationlayer 1911 and the second back passivation layer 1922 using an atomiclayer deposition (ALD) method to simultaneously form the second frontpassivation layer 1912 and the third back passivation layer 1923.

Next, hydrogenated silicon nitride (SiNx:H) is stacked on the secondfront passivation layer 1912 using the PECVD method to form the thirdfront passivation layer 1913.

As described above, because the silicon oxide layers 1911 and 1921 aresimultaneously formed through one process and also the aluminum oxidelayers 1912 and 1923 are simultaneously formed through one process,manufacturing time of the front passivation region 191 and the backpassivation region 192 is reduced.

In the embodiment of the invention, each of the front passivation region191 and the back passivation region 192 has the three layers (i.e.,three component layers). Alternatively, the front passivation region 191may have two layers (i.e., two component layers).

For example, if the front passivation region 191 has two layers, one ofthe first front passivation layer 1911 formed of silicon oxide and thesecond front passivation layer 1912 formed of aluminum oxide may beomitted. In this instance, the first back passivation layer 1921 and thesecond back passivation layer 1922 may be formed using the PECVD method.

Further, the back passivation region 192 may have two layers. In thisinstance, one of the first to third back passivation layers 1921 to 1923may be omitted.

The thicknesses and the refractive indexes of the remaining componentfront passivation layers may be adjusted depending on the omittedcomponent front passivation layer. Further, the thicknesses and therefractive indexes of the remaining component back passivation layersmay be adjusted depending on the omitted component back passivationlayer. Hence, the field passivation effect using the fixed charges andthe anti-reflection effect of light may be maintained even if the frontpassivation layer and/or the back passivation layer are omitted.

Each of the plurality of back surface field regions 172 locally orselectively positioned at the back surface of the substrate 110 is aregion (for example, an n⁺-type region) that is more heavily doped thanthe substrate 110 with impurities of the same conductive type as thesubstrate 110. Thus, each back surface field region 172 has a sheetresistance less than the substrate 110 and has conductivity greater thanthe substrate 110.

As shown in FIG. 1, because the plurality of back surface field regions172 are locally or selectively positioned at the back surface of thesubstrate 110, a non-formation portion of the back surface field regions172 exists at least at an edge of the back surface of the substrate 110and in the middle of the back surface of the substrate 110.

A potential barrier is formed by a difference between impurityconcentrations of a first conductive type region (for example, an n-typeregion) of the substrate 110 and the back surface field regions 172.Hence, the potential barrier prevents or reduces electrons from movingto the back surface field regions 172 used as a moving path of holes andmakes it easier for holes to move to the back surface field regions 172.Thus, the back surface field regions 172 reduce an amount of carrierslost by a recombination and/or a disappearance of the electrons and theholes at and around the back surface of the substrate 110 and acceleratea movement of desired carriers (for example, holes), thereby increasingan amount of carriers moving to the back electrode part 150.

The back electrode part 150 is positioned on the back passivation region192 and includes a back electrode 151 and a plurality of back bus bars152 connected to the back electrode 151.

The back electrode 151 is positioned on the back passivation region 192except in a formation area of the plurality of back bus bars 152.Alternatively, the back electrode 151 may be not positioned at the edgeof the back surface of the substrate 110.

The back electrode 151 includes a plurality of contact portions 155,which sequentially pass through the third to first back passivationpayers 1923 to 1921 and are connected to the plurality of back surfacefield regions 172. Hence, the back electrode 151 is selectively orlocally connected to a portion (i.e., the plurality of back surfacefield regions 172) of the substrate 110 through the plurality of contactportions 155.

Accordingly, as shown in FIGS. 1 and 2, because the back surface fieldregions 172 are positioned at and around the back surface of thesubstrate 110 abutting the contact portions 155, the back surface fieldregion 172 is not positioned at the back surface of the substrate 110between the adjacent contact portions 155.

As shown in FIG. 1, the plurality of contact portions 155 are spacedapart from one another at a predetermined distance (for example, about0.5 mm to 1 mm) therebetween and are connected to the substrate 110.Each contact portion 155 has various cross-sectional shapes such as acircle, an oval, and a polygon.

In the embodiment of the invention, the cross-sectional shape of thecontact portion 155 is the cross-sectional shape obtained by cutting thecontact portion 155 parallel to the flat front surface or the flat backsurface of the substrate 110.

Alternatively, each contact portion 155 may have a stripe shapeelongating in one direction while being electrically connected to thesubstrate 110 in the same manner as the front electrode 141. In thisinstance, the number of contact portions 155 having the stripe shape ismuch less than the number of contact portions 155 having the circle, theoval, or the polygon.

The contact portions 155 collect carriers (for example, electrons)moving from the substrate 110 and transfer the carriers to the backelectrode 151.

Because the back surface field regions 172, which have the conductivitygreater than the substrate 110 due to the impurity concentration higherthan the substrate 110, contact the contact portions 155, the mobilityof carriers from the substrate 110 to the contact portions 155 isimproved.

The back electrode 151 including the contact portions 155 may contain aconductive material (for example, aluminum (Al)) different from thefront electrode part 140. Alternatively, the back electrode 151 maycontain the same conductive material as the front electrode part 140.

The back bus bars 152 connected to the back electrode 151 are positionedon the third back passivation layer 1923, on which the back electrode151 is not positioned. The back bus bars 152 extend in the samedirection as the front bus bars 142 and have a stripe shape. The backbus bars 152 and the front bus bars 142 are positioned opposite to eachother with the substrate 110 between them. In embodiments of theinvention, the back bus bars 152 and the front bus bars 142 may bealigned.

The back bus bars 152 collect carriers transferred from the backelectrode 151, similar to the front bus bars 142. Thus, the back busbars 152 may be formed of a material with conductivity greater than theback electrode 151. For example, the back bus bars 152 contain at leastone conductive material such as silver (Ag).

The back bus bars 152 are connected to the external device and outputthe collected carriers (for example, holes) to the external device.

Unlike the configuration illustrated in FIG. 1, the back bus bars 152may partially overlap the back electrode 151 in another embodiment ofthe invention. In this instance, a contact resistance between the backelectrode 151 and the back bus bars 152 may decrease by an increase in acontact area between the back electrode 151 and the back bus bars 152.Hence, an amount of carriers transferred from the back electrode 151 tothe back bus bars 152 may increase.

The number of back bus bars 152 shown in FIG. 1 may vary, if necessaryor desired.

An operation of the solar cell 11 having the above-described structureis described below.

When light irradiated to the solar cell 11 is incident on the substrate110 through the front passivation region 191 and the emitter region 121,electron-hole pairs are generated in the substrate 110 by light energyproduced based on the incident light. In this instance, because areflection loss of the light incident on the substrate 110 is reduced bythe front passivation region 191 and the textured surfaces, an amount oflight incident on the substrate 110 increases.

The electrons move to the n-type substrate 110 and the holes move to thep-type emitter region 121 by the p-n junction of the substrate 110 andthe emitter region 121. The holes moving to the p-type emitter region121 are collected by the front electrodes 141 and the front bus bars 142and then are transferred to the front bus bars 142. The electrons movingto the n-type substrate 110 are transferred to the contact portions 155and then are collected by the back bus bars 152 via the back electrode151. When the front bus bars 142 are connected to the back bus bars 152using electric wires, current flows therein to thereby enable use of thecurrent for electric power.

In the embodiment of the invention, because the front and backpassivation regions 191 and 192 each having the three layers arerespectively formed on the front surface and the back surface of thesubstrate 110, the passivation effect in the front surface and the backsurface of the substrate 110 is improved. Hence, an amount of carrierslost by defects decreases.

The recombination and/or the disappearance of electrons and holes areprevented or reduced by the field passivation effect using the fixedcharges of the front and back passivation regions 191 and 192. Hence, anamount of carriers moving to the front electrode part 140 and the backelectrode part 150 greatly increases.

The front and back passivation regions 191 and 192 each having theabove-described structure may be applied to various kinds of solarcells.

For example, the front and back passivation regions 191 and 192 eachhaving the above-described structure may be applied to a bifacial solarcell, in which light is incident on both the front surface and the backsurface of the substrate 110.

A bifacial solar cell 12 is described in detail below with reference toFIG. 3.

In the following description, structural elements having the samefunctions and structures as the solar cell 1 shown in FIGS. 1 and 2 aredesignated by the same reference numerals, and a further description maybe briefly made or may be entirely omitted.

As shown in FIG. 3, the bifacial solar cell 12 according to anembodiment of the invention includes an emitter region 121 positioned ata front surface (or a first surface) of a substrate 110, a frontpassivation region 191 including first, second, and third frontpassivation layers 1911, 1912, and 1913 which are sequentiallypositioned on the emitter region 121 in the order named, a plurality offront electrodes (or first electrodes) 141 which pass through the frontpassivation region 191 and are connected to the emitter region 121, aback surface field region 172 a positioned at a back surface (or asecond surface) of the substrate 110, a back passivation region 192including first, second, and third back passivation layers 1921, 1922,and 1923 which are sequentially positioned on the back surface fieldregion 172 a in the order named, and a plurality of back electrodes (orsecond electrodes) 151 a which pass through the back passivation region192 and are connected to the back surface field region 172 a.

The back surface field region 172 a is an impurity region doped bydiffusing impurities of the same conductive type as the substrate 110into the back surface of the substrate 110. The back surface fieldregion 172 a may be formed at the entire back surface of the substrate110 or at the entire back surface of the substrate 110 except an edge ofthe back surface of the substrate 110. Thus, the back surface fieldregion 172 a shown in FIG. 3 is different from the back surface fieldregions 172 shown in FIGS. 1 and 2 in the formation position and thenumber. However, since functions and operations of the back surfacefield region 172 a shown in FIG. 3 are the same as the back surfacefield regions 172 shown in FIGS. 1 and 2, a further description may bebriefly made or may be entirely omitted.

The bifacial solar cell 12 according to the embodiment of the inventionincludes a plurality of front bus bars, which extend in a directioncrossing the front electrodes 141 and are connected to the frontelectrodes 141, and a plurality of back bus bars, which extend in adirection crossing the back electrodes 151 a and are connected to theback electrodes 151 a. Thus, the bifacial solar cell 12 includes a frontelectrode part including the plurality of front electrodes 141 and theplurality of front bus bars and a back electrode part including theplurality of back electrodes 151 a and the plurality of back bus bars.In embodiments of the invention, the front electrodes 141 and the backelectrodes 151 a may be aligned.

Thus, the back electrodes 151 a shown in FIG. 3 are different from theback electrodes 151 shown in FIGS. 1 and 2 in the formation position andthe number. However, since functions and operations of the backelectrodes 151 a shown in FIG. 3 are the same as the back electrodes 151shown in FIGS. 1 and 2, a further description may be briefly made or maybe entirely omitted.

In the bifacial solar cell 12 according to the embodiment of theinvention, because the front and back passivation regions 191 and 192each having the three layers are respectively formed on the frontsurface and the back surface of the substrate 110, a passivation effectin the front surface and the back surface of the substrate 110 isimproved. Further, a field passivation effect using fixed charges of thefront and back passivation regions 191 and 192 is improved.

Furthermore, because light is incident on both the front surface and theback surface of the substrate 110, an amount of light incident on thesubstrate 110 increases. Hence, an effect of the bifacial solar cell 12is further improved.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A solar cell comprising: a substrate of a firstconductive type; an emitter region which is positioned at a frontsurface of the substrate and has a second conductive type different fromthe first conductive type; a front passivation region including aplurality of layers which are sequentially positioned on the emitterregion; a back passivation region which is positioned on a back surfaceopposite the front surface of the substrate and includes three layers; afront electrode part which passes through the front passivation regionand is connected to the emitter region; and at least one back electrodewhich passes through the back passivation region and is connected to thesubstrate, wherein the plurality of layers of the front passivationregion include: a silicon oxide layer which is positioned on the emitterregion and is formed of silicon oxide; an aluminum oxide layer which ispositioned on the silicon oxide layer and is formed of aluminum oxide;and a silicon nitride layer which is positioned on the aluminum oxidelayer and is formed of silicon nitride, wherein the three layers of theback passivation region include: a first back passivation layer which ispositioned on the back surface of the substrate and is formed of siliconoxide; a second back passivation layer which is positioned on the firstback passivation layer and is formed of silicon nitride; and a thirdback passivation layer which is positioned on the second backpassivation layer and is formed of aluminum oxide, wherein the secondback passivation layer is positioned between the first back passivationlayer and the third back passivation layer, wherein the at least oneback electrode includes one back electrode having a plurality of contactportions which locally abut the back surface of the substrate, whereinthe solar cell further comprises a plurality of back surface fieldregions respectively positioned at the back surface of the substrateabutting the plurality of contact portions, wherein the first backpassivation layer is directly on the plurality of back surface fieldregions, the second back passivation layer is directly on the first backpassivation layer, and the third back passivation layer is directly onthe second back passivation layer, wherein the third back passivationlayer is formed as an outermost layer of the three layers of the backpassivation region, and wherein the plurality of layers of the frontpassivation region including the silicon oxide layer, the aluminum oxidelayer, and the silicon nitride layer sequentially positioned on thefront surface of the substrate, and the three layers of the backpassivation region including the first back passivation layer, thesecond back passivation layer, and third back passivation layersequentially positioned on the back surface of the substrate, areasymmetrical to each other.
 2. The solar cell of claim 1, wherein thefirst back passivation layer has a thickness of 5 nm to 30 nm, and thethird back passivation layer has a thickness of 5 nm to 20 nm.
 3. Thesolar cell of claim 2, wherein the first back passivation layer and thethird back passivation layer have the same thickness.
 4. The solar cellof claim 1, wherein a thickness of the second back passivation layer isgreater than a thickness of the first back passivation layer and athickness of the third back passivation layer.
 5. The solar cell ofclaim 4, wherein each of the first back passivation layer and the thirdback passivation layer has the thickness of about 10 nm, and the secondback passivation layer has the thickness of about 90 nm.
 6. The solarcell of claim 1, wherein the first back passivation layer has athickness of 5 nm to 30 nm, the second back passivation layer has athickness of 10 nm to 200 nm, and the third back passivation layer has athickness of 5 nm to 20 nm.
 7. The solar cell of claim 1, wherein thealuminum oxide layer has a thickness of 5 nm to 20 nm, and the siliconnitride layer has a thickness of 10 nm to 200 nm.
 8. The solar cell ofclaim 7, wherein a thickness of the silicon nitride layer is greaterthan a thickness of the aluminum oxide layer.
 9. The solar cell of claim8, wherein the silicon nitride layer has the thickness of about 90 nm,and the aluminum oxide layer has the thickness of about 10 nm.
 10. Thesolar cell of claim 1, wherein the silicon oxide layer has a thicknessof 5 nm to 30 nm.
 11. The solar cell of claim 10, wherein each of thesilicon oxide layer and the aluminum oxide layer has a thickness ofabout 10 nm, and the silicon nitride layer has a thickness of about 90nm.
 12. The solar cell of claim 1, wherein the silicon oxide layer andthe first back passivation layer have the same thickness.
 13. The solarcell of claim 1, wherein the first conductive type is an n-type, and thesecond conductive type is a p-type.
 14. The solar cell of claim 1,wherein the aluminum oxide layer and the third back passivation layerhave the same thickness.
 15. The solar cell of claim 1, wherein thefront electrode part includes a plurality of front electrodes and aplurality of front bus bars connected to the plurality of frontelectrodes at crossings of the plurality of front electrodes and theplurality of front bus bars.
 16. The solar cell of claim 1, wherein thefront electrode part has a lattice shape on the front surface of thesubstrate.
 17. The solar cell of claim 1, wherein the at least one backelectrode further includes a plurality of back bus bars connected to theone back electrode.
 18. The solar cell of claim 17, wherein the one backelectrode is positioned on the third back passivation layer except in aformation area of the plurality of back bus bars.
 19. The solar cell ofclaim 1, wherein the plurality of back surface field regions are notpositioned at the back surface of the substrate between the adjacentcontact portions.
 20. The solar cell of claim 1, wherein the one backelectrode is connected to the plurality of back surface field regionsthrough the plurality of contact portions.